1. Field of the Invention
The present invention relates to GPS-supported inertial attitude and heading reference (INS-Systems) systems having Kalman filters. More particularly, this invention pertains to a method for determination and compensation of scale factor error in such a system due to wavelength changes in multiple-axis fiber optic gyroscopes having a common light source.
2. Description of the Prior Art
DE 196 51 543 C1 teaches subtraction of attitude and heading angles produced by a GPS receiver from the corresponding data supplied by an inertial sensor for iterative correction of a platform calculation by means of a Kalman filter, in a attitude and heading reference system including an inertial system that is assisted by means of a GPS receiver. Such method obtains an accurate attitude/heading reference of comparatively wide bandwidth that is independent of acceleration sensors. Correction models for GPS/INS mechanizations for attitude and heading reference systems in which INS values are corrected by a Kalman filter with different disturbance variables are disclosed in Kayton and Fried, Avionics Navigation Systems, Second Edition (1997), pgs. 72–98.
For single axis, GPS-supported track control systems which are equipped with a gyroscope and Kalman filter correction, U.S. Pat. No. 5,469,158 discloses the scale factor errors caused by external influences (temperature, humidity) on the system electronics and optics being compensated for by a calibration during manufacture and by scale factor control of the gyroscope. A multiple axis track and attitude reference system that is fed from a common light source and, in particular, the scale factor error which follows from a change in the light wavelength are not addressed.
The inertial sensor system with GPS assistance described in U.S. Pat. No. 5,617,317 is primarily intended to improve the heading accuracy with respect to true North, which is considered to be of greater importance than the roll and pitch axes of an aircraft.
It is also generally known to estimate the errors of inertial sensors (i.e., zero error (bias), scale factor and axis alignment error) in INS/GPS systems with the aid of external information using a Kalman filter technique. Such estimated values are then utilized to correct sensor data. Satellite navigation systems, such as the United States Global Positioning System (“GPS”) are particularly suitable as they provide position and velocity virtually continuously, with high accuracy and without drift. Kalman filters make it possible to use the difference between the position and velocity data from the GPS and the inertial system to estimate inertial sensor errors and to thoroughly perform appropriate corrections.
However as mentioned above, the determination of sensor errors with the aid of external assistance and Kalman filter techniques, is dependent upon a specific amount of motion dynamics of the vehicle (e.g. an aircraft) as error mechanisms of the inertial sensors can only be stimulated in this way. They can then be observed via the Kalman filter.
Such vehicles generally move in a horizontal plane, with pitch and roll movements assuming relatively large values for only a short time. On the other hand, GPS reception is lost during very major attitude angle changes (e.g. aircraft engaged in aerobatics) when the GPS antenna can no longer receive satellite signals. As a result, the scale factor can then no longer be estimated or determined satisfactorily. Furthermore, any increase in scale factor error resulting from a change in the wavelength of the light source will become evident only after the systems have been in use for a relatively long time.
The reduction in accuracy in attitude/heading reference systems generally remains unknown as the system error is largely suppressed by vertical and magnetic sensor assistance, and has therefore been found to be relatively insignificant. However, an improvement in scale factor accuracy and its long-term stability is absolutely essential for future tasks, particularly in assisted inertial navigation and when attitude angle accuracy is subject to stringent requirements.